Adhesives based on block copolymer rubbers are commonly used in the construction of disposable absorbent articles (e.g., diapers, training pants, feminine hygiene articles, adult incontinence devices, and the like) and are sometimes referred to as construction adhesives or chassis adhesives. Commonly used chassis adhesives include so-called “hot-melt” adhesives (i.e., adhesives that exhibit fluid or fluid-like behavior when exposed to a particular temperature or range of temperatures), which are typically used to join nonwovens and/or films to each other and/or other absorbent article components. Hot-melt adhesives are typically applied to a nonwoven or film substrate at temperatures above 130° C. and often above 150° C. Traditional hot-melt adhesives are well known in the art and generally have a formulation that includes a polymer for providing cohesive strength; a tackifying resin or analogous material for providing adhesive strength; waxes, plasticizers or other materials for modifying viscosity (i.e., flowability); and/or other additives including, but not limited to, antioxidants or other stabilizers, pigments, and/or fillers.
Applying hot-melt adhesives to a substrate at temperatures of 150° C. or more may be unsuitable for some manufacturing processes that utilize substrates susceptible to thermal damage from the hot-melt adhesive. Thermal damage to the substrate may result in aesthetic and/or structural defects in a finished product (e.g., holes, weakened areas, brittle areas, wrinkled or gathered areas, discolored areas, areas of reduced elasticity and/or extensibility, and combinations of these). Therefore, it may be desirable to apply a hot-melt adhesive to a substrate at a temperature of less than 150° C. or even less than 130° C. to potentially reduce the risk of thermal damage to the substrate. Adhesives that are intended to be applied to a substrate at temperatures less than 130° C. or 150° C. are sometimes referred to as low application temperature adhesives. The use of low application temperature adhesives may reduce the likelihood of thermal damage to a substrate, reduce the energy consumption of process equipment, and/or reduce the thermal aging of process equipment. Using low application temperature adhesives may even provide a safer work environment.
Conventional low application temperature hot-melt adhesives typically use lower molecular weight polymers and/or higher levels of diluent (e.g., waxes or oils) to lower the viscosity of the adhesive. The lower viscosity of conventional low application temperature hot-melt adhesives may be relatively non-problematic when applied to conventional substrates. However, some manufacturers have turned to lower basis weight substrates to realize a cost savings in the face of a highly competitive marketplace. Other manufacturers may use low basis weight substrates to provide a particular consumer benefit (e.g., increased softness). When the lower viscosity adhesive is applied to a low basis weight substrate, undesirable “bleed-through” may occur. That is, the adhesive may pass all the way through the substrate from one side to the other. Such adhesive bleed-through has been known to lead to cosmetic defects in the substrate or article and/or undesirable transfer of adhesive to other articles (e.g., clothing, skin, furniture, packaging). Additional plasticizer may also result in lower bond strength between the substrates due to a corresponding lower ratio of polymer and/or tackifier to plasticizer.
In addition to the problems described above, conventional hot-melt adhesives (including low application temperature hot-melt adhesives) may not be suitable for use with conventional process equipment over a desired range of process conditions. Conventional hot-melt adhesives are typically liquefied, for example, using a hot-melt tank and subsequently transported via a pump to the point of application (e.g., the surface of a substrate). Along the way, the adhesive may pass through a variety of other components and/or be subjected to a variety of process conditions and shear rates. Generally, at the point of application, the substantially liquefied adhesive composition will pass through a nozzle, bank of nozzles, or some other mechanical element such as a slot. In some manufacturing processes, it may be desirable or even necessary to vary the manufacturing line speed, depending on, for example, the size or shape of the articles being produced and/or the number of articles to be produced in a particular amount of time. In order to apply a consistent amount of adhesive to each product being produced, the rate at which the adhesive is applied may need to be decreased or increased to correspond to a slower or faster manufacturing line speed. One way to change the rate at which the adhesive is applied may be to change the application pressure of the adhesive and thereby increase the flowrate of adhesive through the applicator nozzle. However, changing the application pressure of the adhesive may change the shear rate to which the adhesive is subjected, and thus change the shear viscosity of the adhesive. For example, it is known that an increase in shear rate (e.g., due to a corresponding increase in the pressure applied to the adhesive in the adhesive applicator) may cause a phenomenon in certain fluids known as shear thinning (i.e., a decrease in shear viscosity). On the other hand, decreasing the shear rate may have the opposite effect (i.e., the fluid may experience less shear thinning and exhibit a relative increase in shear viscosity). The resultant lower shear viscosity may cause the adhesive to build up on the applicator, for example at the edge of the applicator adjacent to the applicator outlet (e.g., nozzle tip or slot edge). As the adhesive builds up on the applicator, one or more adhesive blobs may fall off the applicator and onto the moving web, causing undesirable contamination of the substrate.
Accordingly, it would be desirable to provide an absorbent article constructed with a low basis weight substrate and a hot-melt adhesive that provides suitable adhesive bond strength and low bleed-through. It would also be desirable to provide an absorbent article constructed with a hot-melt adhesive that has a suitable shear viscosity through a desired range of process conditions. It would further be desirable to provide a process for constructing an absorbent article with a low application temperature adhesive.